9.2.1.2.1 Positive Voltage Doubler

The output characteristics of this circuit can be approximated by an ideal voltage source in series with a resistance. The voltage source equals 2 V+. The output resistance R_OUT is a function of the ON resistance of the internal MOSFET switches, the oscillator frequency, the capacitance and equivalent series resistance (ESR) of C₁ and C₂. Since the switching current charging and discharging C₁ is approximately twice as the output current, the effect of the ESR of the pumping capacitor C₁ will be multiplied by four in the output resistance. The output capacitor C₂ is charging and discharging at a current approximately equal to the output current, therefore, its ESR only counts when in the output resistance. A good approximation of R_OUT is:

Equation 1.

where

• R_SW is the sum of the ON resistance of the internal MOSFET switches shown in Figure 9. R_SW is typically 8 Ω for the LM2765.

The peak-to-peak output voltage ripple is determined by the oscillator frequency, the capacitance and ESR of the output capacitor C₂:

Equation 2.

High capacitance, low-ESR capacitors can reduce both the output resistance and the voltage ripple.

The Schottky diode D₁ is only needed for start-up. The internal oscillator circuit uses the OUT pin and the GND pin. Voltage across OUT and GND must be larger than 1.8 V to insure the operation of the oscillator. During start-up, D₁ is used to charge up the voltage at the OUT pin to start the oscillator; also, it protects the device from turning-on its own parasitic diode and potentially latching-up. Therefore, the Schottky diode D₁ must have enough current carrying capability to charge the output capacitor at start-up, as well as a low forward voltage to prevent the internal parasitic diode from turning-on. A Schottky diode such as 1N5817 can be used for most applications. If the input voltage ramp is less than 10 V/ms, a smaller Schottky diode such as MBR0520LT1 can be used to reduce the circuit size.

9.2.1.2.2 Capacitor Selection

As discussed in Positive Voltage Doubler, the output resistance and ripple voltage are dependent on the capacitance and ESR values of the external capacitors. The output voltage drop is the load current times the output resistance, and the power efficiency is:

Equation 3.

where

• I_Q(V+) is the quiescent power loss of the device; and
• I_L²R_out is the conversion loss associated with the switch on-resistance, the two external capacitors and their ESRs.

The selection of capacitors is based on the specifications of the dropout voltage (which equals I_OUT R_OUT), the output voltage ripple, and the converter efficiency. Low ESR capacitors are recommended to maximize efficiency, reduce the output voltage drop and voltage ripple.

9.2.1.2.3 Paralleling Devices

Any number of LM2765 devices can be paralleled to reduce the output resistance. Each device must have its own pumping capacitor C₁, while only one output capacitor, C_OUT, is required as shown in Figure 11. The composite output resistance is:

9.2.1.2.4 Cascading Devices

Cascading the LM2765 devices is an easy way to produce a greater voltage (a two-stage cascade circuit is shown in Figure 12).

The effective output resistance is equal to the weighted sum of each individual device, shown in Equation 5:

Note that the increasing of the number of cascading stages is practically limited since it significantly reduces the efficiency, increases the output resistance and output voltage ripple.

Figure 12. Increasing Output Voltage by Cascading Devices

9.2.1.2.5 Regulating V_OUT

It is possible to regulate the output of the LM2765 by use of a low dropout regulator (such as LP2980-5.0). The whole converter is depicted in Figure 13.

A different output voltage is possible by use of LP2980-3.3, LP2980-3.0, or LP2980-ADJ.

Note that the following conditions must be satisfied simultaneously for worst-case design:

Figure 13. Generating a Regulated +5-V From +3-V Input Voltage